1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus which forms an image on a recording medium by depositing liquid droplets on the recording medium through nozzles.
2. Description of the Related Art
An inkjet image forming apparatus forms an image on a recording medium by depositing ink on the recording medium through nozzles provided in a print head. Various conventional methods for ejecting ink from the image forming apparatus are known, including: a piezoelectric method in which a diaphragm constituting a part of a pressure chamber (ink chamber) is deformed by the deformation of a piezoelectric element (piezoelectric actuator), thereby varying the volume of the pressure chamber such that when the pressure chamber volume is increased, ink is introduced into the pressure chamber through an ink supply passage, and when the pressure chamber volume is reduced, the ink inside the pressure chamber is ejected through the nozzle as an ink droplet; and a thermal inkjet method in which bubbles are formed by heating the ink, and ink droplets are ejected by the expansion energy generated as the bubbles grow.
When ink droplets are ejected through the nozzles in accordance with such ink ejection methods, a minute ink mist in droplet form may occur. This ink mist has negative charge due to the friction generated upon ejection of the ink droplets. If a nozzle face (ejection face) of the print head has positive charge or the recording medium has negative charge at this time, the resultant electrostatic force causes the ink mist to adhere to the nozzle face of the print head. When the ink mist adhered to the nozzle face contacts the ink droplets ejected from the nozzles, a phenomenon whereby the ink droplets do not travel in the desired ejection direction, or in other words defective ejection, occurs.
Various techniques for preventing ink mist adhesion during ink ejection have been proposed.
Japanese Patent Application Publication No. 63-15754 discloses a technique of removing electricity from the nozzle face of a print head without damaging the nozzle face.
According to this document, an electricity-removing electrode is provided in a standby position facing the nozzle face of the print head, where the print head does not perform an ink ejection operation, rather than a position at which the print head performs the ink ejection operation to deposit ink onto the recording medium. An electricity-removing voltage is applied to the electricity-removing electrode so that the electric field becomes zero or opposite to the initial electric field, and in so doing, ink mist is prevented from adhering to the nozzle face.
However, in this technique, the electricity-removing electrode is disposed in the standby position of the print head, and is therefore unable to remove electricity from the nozzle face while the print head is performing the ink ejection operation. As a result, the ink mist that is generated during ink ejection may adhere to the nozzle face before electricity is removed from the nozzle face in the standby position of the print head, and hence defective ejection through the nozzles may still occur.
Japanese Patent Application Publication No. 2003-341109 discloses a technique for preventing liquid mist (ink mist) from adhering to the edge portion on the rear of the print surface of a recording medium and the inside of the apparatus when ink droplets are deposited onto the edge portions of the recording medium such that no margins remain. According to this document, an electrostatic charging member is provided in an area facing the print head on the rear of the print surface of the recording medium being printed, and static electricity charged to the electrostatic charging member by a static electricity generating member is used to collect liquid mist generated at the edge portion on the rear of the print surface of the recording medium and in the inside of the apparatus.
However, in this technique, the electric field is not adjustable, and hence when the electric field is too strong during ink ejection, the ink droplets ejected through the nozzles may be affected by the electric field. This may result in such adverse effects as so-called oblique ejection, in which the ink droplets travel in an oblique direction, an increase in the ink droplet ejection speed or ejection amount, and an increase in ink mist and so on.